In drilling, it is common to extend the length of the drill string as the drill proceeds by adding individual threaded drill pipe sections to the drill string. The drilling action of the bit at the end of the drill string is usually accomplished by rotating the entire drill string in one direction continuously. The rotation is induced by a drilling unit at the surface which rotates an output shaft threaded to the last section of pipe in the drill string. Typically, the direction of rotation of the output shaft drill is the same direction that makes up the threaded connections between the individual sections of pipe and the rotation of the output shaft is therefore efficiently transferred to the drill pipe at the cutting face.
When the pipe is to be removed from the drill string after the drilling is completed, the output shaft must be operated in a reverse direction to unthread the individual pipe from the drill string. However, in the absence of external forces, it is difficult to control which of the many threaded connections will be the first broken by this reverse rotation.
To avoid this problem, particularly in the field of horizontal drilling, it is typical to provide a drill unit which has a mechanism to move the output shaft along the machine at least the length of a section of the drill pipe. To unthread the upper most section of pipe from the drill string, the output shaft is retracted so that the uppermost section of pipe is contained within the drill unit. The end of the next lower pipe is prevented from rotating with a wrench or similar locking method attached to the drill unit. The output shaft is then rotated in the reverse or unthreading direction while an additional person assists the breaking effort with a handheld pipe wrench. This method is relatively fast, but requires two people. Therefore, a need exists for improved mechanism to assist in breaking out the sections of pipe once the drilling has been completed.
One such device is disclosed in U.S. Pat. No. 5,267,621 issued on Dec. 7, 1993 and assigned to the common assignee of the present application.